In recent years, in order to obtain an electron beam having higher brightness and longer operating life than a thermionic cathode, an electron source (hereinafter referred to as a ZrO/W electron source) employing a cathode made of a needle electrode of tungsten single crystal provided with a covering layer comprising zirconium and oxygen has been used (Non-Patent Document 1).
The ZrO/W electron source is one wherein a diffusing source made of zirconium oxide is provided on a needle cathode of tungsten single crystal having an axial orientation being <100> orientation, so that zirconium and oxygen are diffused to form a covering layer (hereinafter referred to as a ZrO covering layer). By this ZrO covering layer, the work function of the (100) crystallographic plane of tungsten single crystal is reduced to a level of from 4.5 eV to 2.8 eV, and only the very small crystallographic facet corresponding to the (100) crystallographic plane formed at the forward end of the cathode becomes an electron emission area, whereby an electron beam having a higher brightness than by a conventional thermionic cathode can be obtained, and yet this electron source has such a characteristic that it has a longer operating life. Further, such an electron source has characteristics such that it is more stable than a cold field emission electron source and is operable even under a low vacuum degree and thus easy to use (Non-Patent Document 2).
As shown in FIG. 1, in the ZrO/W electron source, a needle cathode 1 of tungsten having <100> orientation which emits an electron beam is fixed by e.g. welding to a predetermined position of a tungsten filament 3 provided on conductive terminals 4 fixed to an insulator 5. A zirconium- and oxygen-diffusing source 2 is formed at a portion of the cathode 1. Although not shown in the drawings, the surface of the cathode 1 is covered with a ZrO covering layer.
The cathode 1 is Joule heated by the filament 3 and used usually at a temperature of about 1,800 K. Accordingly, the ZrO covering layer on the surface of the cathode 1 will be lost by evaporation. However, from the diffusing source 2, zirconium and oxygen will diffuse and will be continuously supplied to the surface of the cathode 1, and consequently, the ZrO covering layer will be maintained.
When the ZrO/W electron source is used, the forward end of the cathode 1 is disposed between a suppressor electrode 6 and an extractor electrode 7 (FIG. 2). To the cathode 1, a high voltage negative against the extractor electrode 7 is applied and to the suppressor electrode 6, a negative voltage at a level of a few hundred volts against the cathode 1 is applied to suppress thermionic electrons from the filament 3.
In a CD-SEM or a wafer inspection apparatus to be used at a low accelerating voltage, the ZrO/W electron is source is operated at an angular intensity of from 0.1 to 0.2 mA/sr for such reasons that the probe current is stable and the energy spread can be suppressed.
On the other hand, in an electron beam lithography apparatus, an Auger electron spectroscope, etc., a throughput is of importance, and accordingly, the electron source is operated at a high angular intensity at a level of 0.4 mA/sr. In such an application where a throughput is of importance, operation at a still higher angular intensity is desired, and an operation at an angular intensity as high as 1.0 mA/sr may sometimes be required.
However, with the ZrO/W electron source, (1) during the operation at a high angular intensity, the upper limit of the angular intensity is at a level of 1.0 mA/sr, and (2) at that time, the extraction voltage applied between the cathode and the extractor electrode is high at a level of at least 4 kV, whereby the field intensity at the forward end of the chip will be remarkably high at a level of from 0.4×109 to 1.0×109 V/m, and the failure frequency due to arc discharge tends to be high (Non-Patent Document 3).
In order to solve such problems, the present inventors have previously proposed an electron source provided with a cathode made of a single crystal of tungsten or molybdenum with its surface covered by a covering layer of oxygen and a metal element selected from Groups 2A, 3A and 4A of the Periodic Table, which has a truncated cone shape with a total conical angle of at least 25° and at most 95° with its upper surface being a flat electron emission surface with a diameter of at least 5 μm and at most 200 μm (Patent Document 1).
Non-Patent Document 1: D. Tuggle, J. Vac. Sci. Technol. 16, p 1699 (1979)
Non-Patent Document 2: M. J. Fransen, “On the Electron-Optical Properties of the ZrO/W Schottky Electron Emitter”, ADVANCES IN IMAGING AND ELECTRON PHYSICS, VOL. III, p 91-166, 1999 by Academic Press.
Non-Patent Document 3: D. W. Tuggle, J. Vac. Sci., Technol. B3(1), p 220 (1985).
Patent Document 1: WO2004/073010A1